The present invention relates to a back lighting device for liquid-crystal panels that illuminates transmissive or semi-transmissive panels from the rear side.
Thin liquid-crystal displays provided with a backlighting mechanism that allows easy viewing of information on the screen are used with recent versions of lap-top or book type word processors and computers. The backlighting mechanism in common use adopts an "edge lighting" method in which a linear light source such as a fluorescent tube is provided at one end portion of a transmissive light conducting plate as shown in FIG. 1.
In order to insure uniform light emission from a plane in the "edge lighting" method, the effective emission length of a fluorescent tube that excludes non-emitting portions such as the base and electrodes may be made greater than the length of an end face that corresponds to the display portion of the light conducting plate. However, this idea involves two major problems; first, the overall size of the device is increased compared to the liquid-crystal display face; secondly, the amount of incident light that cannot be effectively admitted into the light conducting plate increases to reduce the efficiency of power to luminance conversion.
In another approach, a certain method of converging light is applied to the light conducting plate so as to enable the use of a fluorescent tube the length of which is equal to or smaller than the length of an end portion of the light conducting plate. Even if this is possible, the portions of the fluorescent tube near the electrodes have a tendency to become dark as it is turned on and off many times. Thus, in the initial period of use, the backlighting device may produce uniform light emission from a plane but, as a result of prolonged use, the luminance of the light conducting plate in areas where light is guided from the neighborhood of the electrodes will decrease, causing unevenness in the luminance distribution across the surface of the plate.
When back lighting devices are driven with a battery as is often the case today, a further improvement in the efficiency of power to luminance conversion is desired. To meet this need, the end face of the light conducting plate which is the farthest from the linear light source is covered with a light diffusing plate so that no part of the incident light will emerge from that end face by travelling through the plate in the same way as light is guided through optical fibers. This method, however, has had the problem that the luminance of the light emitting surface in the neighborhood of the end face covered with the light diffusing plate becomes so high as to provide an uneven luminance distribution across the emitting surface.